The Thrombolysis in Myocardial Infarction (TIMI) Phase II Trial is designed to determine whether intravenous thrombolytic therapy given in the early hours of an acute myocardial infarction should be followed by elective percutaneous transluminal coronary angioplasty (PTCA). All patients will be treated with recombinant tissue-type plasminogen activator (rt-PA) within four hours of the onset of symptoms of an acute myocardial infarction. In one group of TIMI Clinical Centers, patients will be randomized to rt-PA alone or to rt-PA followed by elective angioplasty which will be performed either at three hours after the start of the infusion of rt-PA or at 18 to 36 hours after the start of the infusion of rt-PA. In all other TIMI Clinical Centers, patients will be randomized to rt-PA alone or rt-PA plus 18 to 36 hours PTCA. In addition, in these latter clinics, all patients who are eligible for the initiation of beta-blocker therapy will be randomized to either immediate or deferred beta-blocker therapy. The primary objective of TIMI Phase II is to determine the effects of thrombolytic therapy alone versus thrombolytic therapy followed within 18 to 36 hours by elective PTCA. The principal endpoint for this comparison is the combined endpoint total mortality and nonfatal myocardial infarction at three months after an acute myocardial infarction. Secondary objectives include the assessment of the combination endpoint at one-year follow-up, and at 10 days post infarction (in-hospital mortality and infarction).